The present invention relates to a method for forming a multi-layer metal wiring of a semiconductor device, and more particularly to a multi-layer metal wiring of a semiconductor device including a diffusion barrier layer for preventing mutual metal diffusion between upper and lower metal wirings mutually contacted and a method for forming the same.
For highly integrated semiconductor devices, high speed device components are required such that the memory cells are formed in a stack structure. Further, the metal wirings carrying electrical signals to each cell are also formed in a multi-layer structure. The metal wiring laid out in a multi-layer structure provides advantageous design flexibility and allows more leeway in setting the margins for the metal wiring resistance, the current capacity, etc.
Generally, aluminum (Al) has been the choice of metal wiring for its superior electric conductivity and the ease of being applied in a fabrication process. However, copper (Cu) is preferred over Al in products operating in faster speeds and requiring lower operational voltages due to high integration of semiconductor devices, because Cu has relatively lower resistance than Al.
However, it is not preferable to apply only Cu as the material for all metal wirings formed in a multi-layer structure due to increased manufacturing expenses and certain unsuitable characteristics that could be present in the highly integrated device components. Along these lines, Cu were used as the metal wiring material in a multi-layer structure when high speed is important, and Al were used when the speed is relatively less important.
Meanwhile, when Cu is used as the lower metal wiring and Al is used as the upper metal wiring in a multi-layer metal wiring structure, a diffusion barrier layer between the lower and upper metal wirings is necessary to prevent metal diffusion between the metal wirings.
Generally, in a multi-layer metal wiring structure having a lower wiring of Cu and an upper wiring of Al, a Ti layer and/or a TiN layer (i.e., either individually or as a stacked layer) were used as the diffusion barrier layer between the lower and upper metal wirings. However, the diffusion barrier layer made of stacked Ti and TiN layers cannot secure the thickness needed to sufficiently suppress the metal diffusion between the metal wirings.
It is not impossible to increase the thickness of the Ti and/or TiN layers to suppress the metal diffusion between the lower Cu wiring and the upper Al wiring; however, this only leads to reduction of the sectional area of the Al upper wiring, which is formed in a damascene pattern, and thereby causes the undesirable increase of the metal wiring resistance.
Also, when the thickness of the stacked Ti and TiN layers is increased, filling Al in a via hole in the damascene pattern to form a upper metal wiring would become more difficult such that voids may be generated in the via hole leading to the significantly increased metal wiring resistance.
Therefore, increasing the thickness of the stacked Ti and TiN layers for purposes of suppressing the metal diffusion between the lower Cu wiring and the upper Al wiring is impractical for use.